what happens to voltage when I add a good choke - diyAudio
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Old 17th July 2013, 04:21 AM   #1
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Default what happens to voltage when I add a good choke

If I add a reasonable-inductance but enormous choke with very little DC resistance before the capacitor filters in the main B+ supply, it should be easier for the power transformer to deliver current thru more of the wall-supply cycle, but what is the end result for the resultant B+ voltage? Will it always drop? I guess I should model but what's the rule of thumb?
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Old 17th July 2013, 04:28 AM   #2
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The L should be between the C's ie CLC. You will get a small drop in voltage.
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Old 17th July 2013, 06:53 AM   #3
Enzo is offline Enzo  United States
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Look up any basic tutorial on power supplies and read the differences between capacitor input filter versus choke input filter. The cap input filter charges to peak, or about 1.414 times RMS voltage of the AC. The choke input filter will charge to about 0.9 times RMS.

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Old 17th July 2013, 09:23 AM   #4
DF96 is offline DF96  England
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An ideal choke input supply gives you a DC voltage equal to 2 sqrt(2)/pi of the secondary AC RMS voltage. This is 90.03%. The derivation requires calculus and trigonometry - the maths you learnt in school does have its uses!
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Old 17th July 2013, 01:56 PM   #5
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Quote:
Originally Posted by DF96 View Post
An ideal choke input supply gives you a DC voltage equal to 2 sqrt(2)/pi of the secondary AC RMS voltage. This is 90.03%. The derivation requires calculus and trigonometry - the maths you learnt in school does have its uses!
So does building it and bunging a meter on it!

Enzo already said 'about 90%'
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Old 17th July 2013, 02:07 PM   #6
DF96 is offline DF96  England
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A meter will tell you what it is doing, but only after you have built it. A meter won't tell you why it is doing it, or what to change to make it do something different.
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Old 24th July 2013, 03:42 AM   #7
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Thanks! Very useful rule-of-thumb. ...and thanks especially for not ridiculing my ignorance.
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Old 24th July 2013, 04:01 AM   #8
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Thanks again Enzo, I remember reading that link from aikenamps.com before but apparently neither the math nor rule of thumb stuck in my brain. Unlike what Katie & dad posted, a CLC is still basically a capacitor input filter, and a choke input filter like I asked about has a choke immediately after the rectifier. But either way, I'm not talking about a small choke between B+ and grid supply & preamp either.

I'm cooking up another single-ended guitar amp and picking out a power transformer, and I have some very nice chokes. So I'm just deciding what to do with them, if anything.
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Old 24th July 2013, 06:23 AM   #9
Enzo is offline Enzo  United States
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That is why they are called capacitor INPUT filter or choke INPUT filter. it is about what the rectified AC sees first. What comes after a cap doesn't change that.
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Old 24th July 2013, 10:19 AM   #10
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Originally Posted by cyclecamper View Post
Thanks again Enzo, I remember reading that link from aikenamps.com before but apparently neither the math nor rule of thumb stuck in my brain. Unlike what Katie & dad posted, a CLC is still basically a capacitor input filter, and a choke input filter like I asked about has a choke immediately after the rectifier. But either way, I'm not talking about a small choke between B+ and grid supply & preamp either.

I'm cooking up another single-ended guitar amp and picking out a power transformer, and I have some very nice chokes. So I'm just deciding what to do with them, if anything.
Chokes were used historically to reduce hum, because decent sized capacitors weren't available, so there's little reason to use them these days - unless you're going for the extra distortion as the HT rails comes down lower?.
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